Aging a flock of stars in the Wild Duck Cluster

An image of the Wild Duck Cluster was captured by the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile. The blue stars at the center of the image are the stars of the cluster. Every star in the Wild Duck Cluster is roughly 250 million years old. Older, redder stars surround the cluster. Credit: European Southern Observatory

Do star clusters harbor many generations of stars or just one? Scientists have long searched for an answer and, thanks to the University of Arizona's MMT telescope, found one in the Wild Duck Cluster, where stars spin at different speeds, disguising their common age.

In a partnership between the UA and the Korean Astronomy and Space Science Institute, a team of Korean and Belgian astronomers used UA instruments to solve a puzzle about flocks of stars called open clusters.

Astronomers have long believed that many open clusters consist of a single generation of stars because once stars have formed, their radiation blows away nearby material needed to make new stars. But in the Wild Duck Cluster – known by scientists as Messier 11, or M11 – stars of the same brightness appear in different colors, suggesting they are of different ages. Unless scientists had missed important clues about stellar evolution, there had to be another explanation for the spread of colors in this accumulation of about 2,900 stars.

"Astronomers have been working on this question for decades," said Serena Kim, an associate astronomer at the UA's Steward Observatory. "Do clusters form in one generation or multiple generations? Our study answered this question for the Wild Duck Cluster."

Beomdu Lim of Kyung Hee University led an international team of astronomers who used the MMT telescope – jointly operated by the UA and the Smithsonian Astrophysical Observatory – to study the cluster. The team discovered that it is not the stars' ages that cause them to appear in a spread of color: it is their rotation.

Open clusters contain thousands of stars that astronomers hypothesize formed from the same giant clouds of gas. These stars come in all sizes, from short-lived, giant blue stars dozens of times more massive than our sun, to long-lived low-mass dwarves that will burn for 10 billion years or longer. The brightness and color of each star changes as it grows older, allowing scientists to determine its age.

"As a star is getting older and older, it brightens and becomes redder," Lim said.

The MMT Telescope is located on Mount Hopkins, 47 miles south of Tucson. When the telescope was completed in 1979, it was called the Multiple-Mirror Telescope, as it was comprised of six smaller mirrors. The smaller mirrors were replaced by a single 6.5-meter mirror in 2000, but the name MMT was retained. Credit: Courtesy of the MMT Observatory

Astronomers plot young stars' brightness and color in a diagonal line – from bright, blue and massive at the top of the line, down to faint, red and less massive at the bottom – called the main sequence.

The turning point – the point at which a star ages and veers off the main sequence – is used to determine the age of clusters based on the known life expectancy of each star. If the stars leave the main sequence at the same point, like cars on a freeway taking the same exit, then the stars of the cluster are all the same age.

In the Wild Duck Cluster, however, the stars veer off the diagonal at different points, like cars taking different exits along a freeway.

"This does not seem intuitive, since the stars in an open cluster like M11 are thought to belong to the same generation," Kim said.

Lim and his team set out to discover what stellar properties could potentially explain this pattern.

They turned the MMT telescope toward the cluster to examine the color spectrum of the stars using an Hectochelle. The instrument acts like a prism and spreads starlight into its components, which astronomers call a spectrum. The spectra are like barcodes, with each line identifying a different chemical in the star's makeup.

Hectochelle can capture detailed spectra of many stars at once, making it an ideal instrument to observe clusters like the Wild Duck, which consist of thousands of stars.

A plot comparing the brightness (on the y-axis) to the color (on the x-axis) of 250-million year-old stars in the Wild Duck Cluster. The blue dots indicate individual stars. The bluest stars are on the left side, and the reddest stars are on the right side. The red line indicates the path across this plot that stars take over the course of their lifetime. Credit: Beomdu Lim

As a star rotates, one side of it is moving toward the Earth and the other is moving away. The half of the star rotating toward the Earth emits light with wavelengths that look squished, making the light look bluer than it would be if the star were not moving. The half of the star rotating away from the Earth causes the wavelengths to look stretched, making its light seem redder. This squishing and stretching causes spectral lines to spread across a range of wavelengths, rather than spiking at just one.

The stars in the Wild Duck Cluster, it turns out, are spread out in the color spectrum not because of different ages, but because of different rotational periods.

"The effects of rotation on stellar evolution were often neglected in the past," said Yaël Nazé, an astronomer at the University of Liège in Belgium and co-author of the paper.

The spectra also revealed that the stars are spinning at different rates. Lim and his team performed computer simulations to find out how fast each star is rotating.

"A rapidly rotating star can remain in the main sequence stage longer than a slowly rotating star," Lim said. "A wide range of velocities of stars results in differences of lifetimes among the stars."

Rotational speed is like a fountain of youth to a star: The faster it spins, the better it mixes hydrogen – the star's fuel – into its core. The more hydrogen the core receives, the longer the star lives, causing it to appear redder than younger siblings.

Stars in the cluster appear in different colors because the cloud they were born in set them into motion that would extend the lifetime for some of them.

Though not a part of the Wild Duck Cluster study, Kim has worked with Lim in the past to study other star clusters and uncover mysteries of star formation. Their collaborations are part of a growing partnership between the UA and the Korean Astronomy and Space Science Institute.

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24 comments

The rotation of the star and its effects: my article inAmerican Journal of Astronomy and Astrophysics 3011059Introduction "… I consider the rotation of objects as the central process which creates the systems of stars, galaxies, the clusters of galaxies, Universe, Multiverse,... ; it creates all systems, determines their appearance and, related to stars, their temperatures, radii, colors, orbital speeds of the objects around a star, their numbers, asteroid belts and gas disks."

Planetary bodies/moons whos atmospheres are emitting EM radiation due to bombardment by cosmic rays. Our Moon radiates gamma, x-ray, EUV etc. The emanations from these distant objects are then attenuated by known processes on their way to us.

@fourinfinities, the Hectochelle is a multi-object Echelle spectrograph that splits the spectra among optical fibers for multiple spectra to be taken at once. You can get more data on it but unless you're an astronomer it won't make much sense to you.

@Solon, the images were taken with a spectrograph. All the things you spoke of have different spectral lines than stars. Care to try again, this time after actually learning some astronomy, not to mention astrophysics?

There are photos, ... ... ... You have to be serious. If he does not see, let him go to the eye doctor. We are interested in evidence and only evidence (stories and beliefs should be left for other areas).The point of my comment was that the authors tried to steal the already published facts. Now they (as they say) "Our study answered this question .. The team discovered that it is not the stars' ages that cause them to appear in a spread of color: it is their rotation." They discovered. already published from 2004 to 2018.With us, they rank it in theft of copyright.

So the Apollo astronauts who said they could see no stars from cislunar space, it is totally black out there,...

As you would know by now, I am more objective/fairminded than most, if not all, posting here, yes? So please take it from me, that the HUBBLE TELESCOPE and the SOLAR OBSERVATION SATELLITES have for decades been proving we CAN 'see' the sun/stars/galaxies LIGHT IN MANY FREQUENCIES while in space. So I advise you as a friend would: please drop this patently untenable/illogical claim of yours, mate! :)

What, in the name of all that is scientific, would make a person believe that something that emits a blackbody spectrum, cannot be seen? Have a look at a BB spectrum. It includes visible wavelengths. It is called 'visible' for a reason.I swear that there is a cabal of 12 year olds infesting this place. The idiocy hurts.

" So please take it from me, that the HUBBLE TELESCOPE and the SOLAR OBSERVATION SATELLITES have for decades been proving we CAN 'see' the sun/stars/galaxies LIGHT IN MANY FREQUENCIES while in space."

Your eyes will see nothing of what Hubble or other space based instruments will see, but you need to understand these devices and how the images are created. The Nikoin DX cameras that have been used up till now on the ISS or shuttle missions will see nothing if they could be pointed away from Earth.Perhaps those who don't believe the astronauts are Moon hoaxers?https://www.youtu...TQ99bMrw

"This does not seem intuitive, since the stars in an open cluster like M11 are thought to belong to the same generation," Kim said.

Again, an assumption that they formed simultaneously from condensation. Nonsense. If they were born from central core star ejections of newly formed matter therein over a long period of time, some stars of the cluster would be old and some would be young. But astronomers are a persistently ignorant bunch, bent on merger mania and their favorite Huge Bang Fantasy.

Magic to you and your fellow maniacs, no doubt! With no real interest in fundamental understanding of nature, you are left with only casting insults in the face of mounting contrary observational evidence. Go on, get with it. What a relief you must feel.